[PATCH stable] tests: use an absolute path to get around '..' being invalid on a dead CWD

[Augie Fackler]

> On Jan 19, 2017, at 4:44 PM, Sean Farley <sean at farley.io> wrote:
> 
> I guess so ... crazy freebsd.

I had another thought on the way home: this might only be a problem in a jail - it’s not the first bit of strange behavior we’ve caught with hg when running in my buildbot-in-a-jail.

In any event, I think this fixes the last test failure on the BSD builder.